STM32 H7X: cleanup setting of power supply and voltage scaling

[GeorgeCGV]

Currently, the H7 configures power supply and voltage scaling in multiple locations: SoC initialization and clock initialization. Different scaling factors are applied: SoC init sets it to 1, while clock init applies the boost scale of 0.

This setup functions correctly when the firmware boots without MCUboot, following the sequence: 3 (default) → 1 → 0. However, when used with MCUboot, the sequence becomes 3 → 1 → 0 → 1 → 0. If the bootloader configures the system clock to 480 MHz and jumps to the application with a scaling factor of 0, the application then sets it back to 1 during early SoC initialization. This results in an incorrect scaling factor for the configured system clock, with potentially unknown consequences.

Additionally, the power source is redundantly set in multiple places to the same value, whereas it could be configured just once in early SoC initialization.

This PR simplifies the process by setting the power source only once in early SoC initialization while leaving voltage scaling to the clock initialization phase.

It has been tested successfully on the H730, but further testing, especially on H7 configurations with both M7 and M4 cores, would be valuable.

[erwango]

It has been tested successfully on the H730, but further testing, especially on H7 configurations with both M7 and M4 cores, would be valuable.

@djiatsaf-st Can you help testing this on stm32h747i_disco (m7 and m4) ?

[erwango]

This code only applies for M7 as no clock config is done in M4 case, so this change is fine. Thanks!